This invention relates to a keyboard apparatus for an electronic musical keyboard instrument, and especially to a technique for producing an appropriate sound with respect to the speed of key depression.
Among conventional keyboard apparatus for an electronic musical keyboard instruments, a keyboard apparatus 101 shown in FIG. 9 is known. The keyboard apparatus 101 includes: a plurality of keys 103 (only one key 103 having a white key 103a and a black key 103b is shown in the drawing); a lower chassis 102a; a rear chassis 102b; a plurality of hammers 104 (only one hammer 104 is shown), and a plurality of adjustment screws 114.
The lower chassis 102a includes a balance rail 115 and a plurality of balance pins 112. The balance rail 115 is secured to the central portion of the top surface of the lower chassis 102a in the front-and-rear direction. The plurality of balance pins 112 is aligned in the left-to-right direction and stand on the balance rail 115.
In the central portion of the key 103, a balance pin hole 103e is formed. By inserting the balance pin 112 through the balance pin hole 103e, the key 103 is supported by the balance pin 112 and the balance rail 115 such that the front portion of the key 103 are swingable in the up-and-down direction.
The hammers 104 are disposed above the keys 103 so as to respectively apply loads thereto, and extend forward approximately in a linear manner. In the rear end portion of the hammer 104, an arc-shaped shaft hole 104c, which is open backward, is formed. By the shaft hole 104c being engaged with a fulcrum shaft portion 102c of the rear chassis 102b, the hammer 104 is supported by the rear chassis 102b such that the front portion of the hammer 104 is swingable in the up-and-down direction. To the hammer 104, the adjustment screw 114 is attached in a position in vicinity of the shaft hole 104c disposed on the lower surface of the hammer 104.
The adjustment screw 114 includes a base portion constituted with a longitudinal shaft having a head portion 114a in one end, and a male thread portion 114b in another end. The head portion 114a includes a slidable curved surface 114c (see FIG. 11A), having one portion of a spherical body, and an edge portion 114d extending around the circumference of the slidable curved surface 114c. The male threaded portion 114b is engaged with a female thread portion provided to the hammer 104. The adjustment screw 114 is attached to the hammer 104 such that the height thereof can be adjusted. The hammer 104 is disposed on the top surface of the rear end portion of a corresponding key 103 such that the head portion 114a of the adjustment screw 114 abuts on the flat top surface of the sliding receiving member 103f provided on the top surface of the rear end portion of the key 103.
In a period, from when the front portion of the key 103 is not depressed (see FIG. 9) and until when the front portion of the key 103 begins to be depressed (to be referred to as an initial phase of key depression), as shown in FIG. 11A, the longitudinal shaft of the adjustment screw 114 is disposed approximately in parallel with respect to the vertical direction of the top surface of the sliding receiving member 103f. In this phase, the slidable curved surface 114c of the adjustment screw 114 slides on the top surface of the sliding receiving member 103f approximately in the central portion of the curved surface 114c. This sliding portion of the slidable curved surface 114c is to be referred to as the central portion 114f. In a state wherein the key 103 is depressed to a maximum inclined angle, to which the key 103 can be inclined at most, (to be referred to as the final phase of key depression, see FIG. 10), as shown in FIG. 11B, the longitudinal shaft of the adjustment screw 114 is inclined backward with respect to the vertical direction of top surface of the sliding receiving member 103f. In this phase, the head portion 114a of the adjustment screw 114 slides on the top surface of the sliding receiving member 103f in the rear portion of the edge portion 114d of the head portion 114a. This sliding portion of the edge portion 114d is to be referred to as a rear edge portion 114h. That is, when the front portion of the key 103 is depressed, the slidable curved surface 114c of the head portion 114a provided to the adjustment screw 114 slides on the top surface of the sliding receiving member 103f in a portion between the central portion 114d and the rear edge portion 114h of the curved surface 114c. 
The rear chassis 102b extends in the left-to-right direction so as to be able to support all of the hammers 104. The rear chassis 102b is connected to the lower chassis 102a, and secured to a key bed (not shown) by a screw (not shown). The rear chassis 102b also extends to the up-and-down direction, and includes a substrate attachment portion 102e extending from the top end of the rear chassis 102b in an inclined manner toward the upper front side. To a leading end portion of the substrate attachment portion 102e, a stopper 109 is disposed so as to restrict an upward swinging movement of the hammers 104. As well as the rear chassis 102b, the stopper 109 extends in the left-to-right direction so as to be provided for all of the hammers 104. Above the hammers 104, a key switch 105 is disposed so as to detect key depression state regarding each of the keys 103.
The key switch 105 is connected to a control device (not shown) which controls sound production of an electric piano. The key switch 105 includes a first contact and a second contact. The first contact is turned on when the front portion of the key 103 is depressed until the key 103 is inclined to a predetermined first inclined angle. The second contact is turned on when the front portion of the key 103 is further depressed until the key 103 is inclined to a predetermined second inclined angle which is larger than the first inclined angle. When the front portion of the key 103 is depressed, a length of time between when the first contact is turned on and when the second contact is turned on is measured. As a result, a velocity (sound volume) is obtained depending on the key depression speed.